Abstract. Observations of CO2 were made in the upper
troposphere and lower stratosphere in the deep tropics in order to determine
the patterns of large-scale vertical transport and age of air in the
Tropical Tropopause Layer (TTL). Flights aboard the NASA WB-57F aircraft
over Central America and adjacent ocean areas took place in January and
February, 2004 (Pre-AURA Validation Experiment, Pre-AVE) and 2006 (Costa
Rice AVE, CR-AVE), and for the same flight dates of 2006, aboard the Proteus
aircraft from the surface to 15 km over Darwin, Australia (Tropical Warm
Pool International Cloud Experiment, TWP-ICE). The data demonstrate that
the TTL is composed of two layers with distinctive features: (1) the lower
TTL, 350–360 K (potential temperature(θ); approximately
12–14 km), is subject to inputs of convective outflows, as indicated by
layers of variable CO2 concentrations, with air parcels of zero age
distributed throughout the layer; (2) the upper TTL, from θ=~360 K to ~390 K (14–18 km), ascends slowly and ages uniformly,
as shown by a linear decline in CO2 mixing ratio tightly correlated
with altitude, associated with increasing age. This division is confirmed by
ensemble trajectory analysis. The CO2 concentration at the level of
360 K was 380.0(±0.2) ppmv, indistinguishable from surface site values
in the Intertropical Convergence Zone (ITCZ) for the flight dates. Values
declined with altitude to 379.2(±0.2) ppmv at 390 K, implying that air
in the upper TTL monotonically ages while ascending. In combination with the
winter slope of the CO2 seasonal cycle (+10.8±0.4 ppmv/yr), the
vertical gradient of –0.78 (±0.09) ppmv gives a mean age of
26(±3) days for the air at 390 K and a mean ascent rate of 1.5(±0.3) mm s−1. The TTL near 360 K in the Southern Hemisphere over
Australia is very close in CO2 composition to the TTL in the Northern
Hemisphere over Costa Rica, with strong contrasts emerging at lower
altitudes (<360 K). Both Pre-AVE and CR-AVE CO2 observed unexpected
input from deep convection over Amazônia deep into the TTL. The CO2
data confirm the operation of a highly accurate tracer clock in the TTL that
provides a direct measure of the ascent rate of the TTL and of the age of
air entering the stratosphere.